Digital coherent detection (DCD) has recently emerged as an attractive technology for high-speed optical receivers by allowing for the access of both amplitude and phase information of a received optical signal, thereby enabling digital compensation of transmission impairments such as dispersion and Polarization Mode Dispersion (PMD). FIG. 1 illustrates a conventional digital coherent receiver 100. Optical Local Oscillator (OLO) 110 generates reference source R. Signal S and reference source R are provided to the input ports of a six-port Polarization Diversity-Optical Hybrid 120. In principle, the hybrid consists of linear dividers and combiners interconnected in such a way that four different vectorial additions of the reference source and the signal to be detected are obtained. For example, a polarization-diversity optical hybrid may be constructed by two polarization beam splitters (PBS's) and two optical hybrids. Each optical hybrid may be based on a Michelson interferometer or Michelson interferometer-like structure. For optical coherent detection, the six-port polarization-diversity optical hybrid mixes the incoming signal S with the reference source R to obtain four mixed signals, (Sx+Rx), (Sx+jRx), (Sy+Ry), and (Sy+jRy), where Sx and Sy are the optical fields of the two orthogonal polarization components of signal S, Rx and Ry are the optical fields of the two orthogonal polarization components of reference R, and j is the imaginary unit. The power waveforms of the four output mixed signals may then be detected by single-ended detectors (SD's) 125 and provided to Analog-to-Digital Converters (ADC) 130. The resultant digital domain signals Ix(y) and Qx(y) are related to the in-phase (I) or real and the quadrature (Q) or imaginary components of each of the two polarization components of the signal S as, Ix(y) ∞ C+real(Sx(y)), and Qx(y) ∞ C+imag(Sx(y)), where C is a constant. These digital signals are provided to a Digital Signal Processor (DSP) 140 for further processing. By applying suitable signal processing algorithms, the amplitude and phase of the unknown incoming signal S can be determined.
The polarization-diversity optical hybrid can be a ten-port device with two input ports and eight output ports, as shown in FIG. 2. The ten-port polarization-diversity optical hybrid 220 mixes the incoming signal S with the reference source R to obtain four pairs of mixed signals (or eight mixed signals), (Sx±Rx), (Sx±jRx), (Sy±Ry), and (Sy±jRy). The power waveforms of each pair of the output mixed signals may then be detected and compared by a balanced detector (BD) 225 and provided to an ADC 130. The resultant four digital domain signals Ix(y) and Qx(y) are related to the in-phase (I) or real and the quadrature (Q) or imaginary components of each of the two polarization components of the signal S as, Ix(y) ∞ real(Sx(y)), and Qx(y) ∞ imag(Sx(y)). These digital signals are provided to a Digital Signal Processor (DSP) 240 for further processing to determine the amplitude and phase of the unknown incoming signal S.
Future optical transport networks will demand increased signal data rates. However, with the anticipated future demand for higher signal data rates (e.g., 1-Tb/s), DCD is expected to be limited by the speed of ADCs. One way to avoid this bottleneck is to use multiple (e.g., 10×) sub-channels at different wavelengths to carry the ultrahigh-speed signal. At the receiver side, a Wavelength De-Multiplexing (W-DMUX) filter is first used to separate the sub-channels. A conventional digital coherent receiver may then be used for each of the sub-channels. In other words, such an approach proposes use of multiple (e.g., 10×) optical local oscillators (OLO's) and optical hybrids, each followed by four detectors and four ADCs, to receive the signal. Furthermore, to allow the receiver to be able to receive any multi-wavelength signal in the commonly used C-band (between about 1530 nm and 1565 nm), the W-DMUX needs to have about 80 output ports if the sub-channel spacing is 50 GHz, and for a different multi-wavelength signal, a different subset of the output ports needs to be physically connected to the optical hybrids. In addition, some such solutions utilizing multiwavelength signals require the local oscillators for all channels to be phase locked and parallelization of the post-compensation scheme to cover the entire spectrum of interest. Unfortunately, the complexity, cost, and inflexibility associated with these receiver approaches are quite high.